1. Field of the Invention
The present invention relates to an apparatus for playing a video signal and more particularly to an apparatus for playing a video signal recorded simultaneously on two recording tracks over a long period of time using a pair of heads spaced from each other by a distance corresponding to one track pitch, in other words, recorded by an apparatus capable of high density video recording.
2. Description of the Related Art
The D2 format is a conventional recording method for digitally recording composite signals used in broadcasting equipment. In this kind of D2 format digital VTR, which is disclosed in U.S. Pat. No. 4,549,227, an analogue video signal is divided up into a plurality of digital channel signals. It is then possible to record these at a high density by bringing them into contact with a number of tracks and changing the azimuthal angle between tracks. A detailed description of this D2 format will now be given. In FIG. 1, a two channel recording method is shown where two channels are recorded in one scan. In FIG. 2, a single segment S is formed on a pair of tracks (channels), with three segments S0, S1, and S0 then making up a field.
An analogue signal is converted into a digital signal in order to get the necessary digital video signal data. After each segment S of this data, that is to say every third of a field as is shown in FIG. 2, has then been shuffled and undergone parity correction, the data is recorded onto the two channels divided into odd and even numbers, so as to to have interpolation within the fields. Also, four channels of digital audio signal data A1 to A4 are recorded at both ends of the video track. As can be seen from FIG. 1, these four channels are recorded two at each end of a recorded video track, and then repeated, two at each of the opposite two ends of the next recorded video track. This "double recording" is to provide data protection in case of damage to the edges of the tape. The longitudinally formed analogue channel is then used as the cue channel or the controller ID (timecode) channel and the actual recording has no guard band.
In this kind of D2 format digital VTR, the rotating head is used for both recording and playing, with two groups of heads being arranged respectively at 180 degrees in pairs shifted the width of one track apart from each other. The recording and playing heads are also shifted at an angle of 90 degrees about the drum axis away from each other.
In the kind of D2 format digital VTR adopting the two channel recording method described above, when recording over long periods of time, that is to say, in the case of high density recording, the recording method can be considered as a method where the overall track width has been uniformly reduced. In order to do this, the rear side head of the pair of heads for two channel recording, namely the head for recording the track at the upper rear side with respect to the recording track, has to be constructed so that its height can be changed by sliding it. Also, as the change in the width of the heads has to be very precise, separate heads have to be used for normal operation and operation over long periods of time.
It follows that, when using a recording method where, as described above, the overall track width has to be uniformly reduced to enable recording to take place over long periods of time, requirements such as the heads being movable and a separate head and changeover function being required complicate the overall construction of the apparatus. In addition to this, within this normal range the track width and the error rate are indirectly proportional to each other, so that the S/N ratio will be reduced. This cannot fail to have a detrimental effect on the sound and picture quality. An enhancing circuit must therefore be added to prevent these detrimental influences on the S/N ratio, which means that the overall scope and size of the circuit for signal processing will increase.
Since this invention sets out to solve the problems described above, its purpose is to provide a video signal recording apparatus which can record over long periods of time simply by changing the speed of movement of the magnetic tape, without the signal to noise ratio being reduced.